1. Field of the Invention
The present invention relates to an image forming system in which at least one sheet feeding apparatus is connected to an image forming apparatus, and further relates to the image forming apparatus and the sheet feeding apparatus.
2. Description of the Related Art
A conventional image forming system including an electrophotographic image forming apparatus such as a printer and a copier for office use has the following configuration. When a sheet feeding apparatus is connected to the image forming apparatus as an additional apparatus for feeding sheets, a CPU in the image forming apparatus is configured to directly control conveyance controls of the sheet feeding apparatus.
However, as the image forming apparatus and the additional apparatus grow into large scale, the image forming system such as a printing machine needing to convey sheets at a fast speed in the apparatus and between apparatuses has various additional apparatuses connected to the image forming apparatus. Therefore, when the additional apparatus is directly controlled, an operational performance of the CPU needs to be greatly improved, thus resulting in a disadvantageous cost.
Further, the length of a harness for the direct control becomes very long according to the scale of the apparatus, which reduces the reliability of the system with regard to malfunction caused by EMI (Electromagnetic Interference) and ease of maintenance, thus also resulting in a disadvantageous cost.
Taking the above into consideration, the following configuration has been made: each of the additional apparatus, e.g., a sheet feeding apparatus, has a CPU for controlling the apparatus itself, and this CPU and the CPU in the image forming apparatus are connected by a serial bus and the like, so that the harness is simplified, and a state of each of the apparatuses is notified to each other via the serial bus. In this way, controlling the image forming apparatus without improving the performance of the CPU has been a generally used technique.
However, in the image forming system in the field of printing, a faster speed and a higher productivity is required in sheet conveyance. For a shorter conveyance interval between sheets and a faster conveyance speed of sheets, when a feeding timing is notified from the sheet feeding apparatus using the serial bus, the sheet feeding interval varies according to a delay amount of a communication delay, thus reducing the productivity in the sheet conveyance. Further, if a timing margin is increased to allow for error in timing caused by the communication delay that is not necessarily constant, the original accuracy in the conveyance would be lost. As a result, there is a possibility that a positional shift of an image, a jam, and the like may occur in the image forming apparatus arranged downstream.
In relation to this, Japanese Laid-Open Patent Publication (Kokai) No. 2002-287578 discloses an image forming apparatus to which a plurality of feeding units can be connected, wherein each of the feeding units and a printer unit has its own CPU, and the feeding units and the printer unit are connected via a bus (for example, a serial bus). In this image forming apparatus, control signal lines of the feeding units carrying a driving load, a sensor input, and the like are put together into the bus, so that a harness is simplified. However, a control of feeding and conveyance affects a timing of an image formation performed by the image forming apparatus arranged downstream. Therefore, each timing signal indicating a start of feeding operation is individually arranged, separately from the bus, between the feeding unit and the printer unit so as not to reduce an accuracy of a timing control in a feeding operation.
However, the following problems arise when the constitution of the above conventional image forming apparatus is applied to the above image forming system in the field of printing: a conveyance path used during a period from a timing when a timing signal notifies the sheet feeding apparatus of a start of feeding operation to feed a sheet from a sheet container to a timing when the fed sheet is passed to the image forming apparatus becomes very long. Therefore, as a result, a conveyance delay and the like caused by slipping of rollers during the feeding and conveyance makes it difficult for the sheet feeding apparatus to accurately pass a sheet to the image forming apparatus at a suitable time with respect to a timing of an image formation in the image forming apparatus.
Further, when the image formation is performed by the image forming apparatus without taking any measure, a timing error becomes large, and there is a possibility that a jam and a positional shift of an image occur. Further, it is necessary to give a timing signal for a feeding operation to each of the sheet feeding apparatuses. In this case, since the image forming apparatus and the sheet feeding apparatuses themselves are large scale, the signal lines are very long although the number of signal lines are a few, and therefore, the signal lines tend to be affected by a malfunction caused by EMI, thus reducing the reliability of the apparatus.
Further, when an abnormal condition occurs in a feeding start timing signal serving as a starting point of a feeding operation, such as breaking of a wire, short-circuit with a casing metal plate, and contact failure of a connector, the starting point of the feeding control is lost, and there is a possibility that the entire image forming system stops in an error state. Preventing interference between a signal wiring path and an edge of the casing metal plate and strengthening an insulation coating of the signal line can be considered as measures for preventing such abnormal conditions of the signal, but such measures result in increasing the cost.